The present invention relates generally to optics, and more particularly, to rhombic-QPSK (quadrature phase shift key) modulation for improved tolerance to nonlinearity in fiber communication systems with co-propagating intensity-modulated channel.
The following references are discussed in this application for background information or improved understanding of existing knowledge.
[1] Alberto Bononi, Marco Bertolini, Paolo Serena, and Giovanni Bellotti, “Cross-Phase Modulation Induced by OOK Channels on Higher-Rate DQPSK and Coherent QPSK Channels,” J. Lightwave Technol. 27, 3974-3983 (2009).
[2] Griffin, R. A.; Carter, A. C.; “Optical differential quadrature phase-shift key (oDQPSK) for high capacity optical transmission,” Optical Fiber Communication Conference and Exhibit, 2002. OFC 2002, vol., no., pp. 367-368, 17-22 Mar. 2002.
[3] Chris R. S. Fludger, Thomas Duthel, Dirk van den Borne, Christoph Schulien, Ernst-Dieter Schmidt, Torsten Wuth, Jonas Geyer, Erik De Man, Giok-Djan Khoe, and Huug de Waardt, “Coherent Equalization and POLMUX-RZ-DQPSK for Robust 100-GE Transmission,” J. Lightwave Technol. 26, 64-72 (2008).
[4] C. Laperle, B. Villeneuve, Z. Zhang, D. McGhan, H. Sun, and M. O'Sullivan, “Wavelength Division Multiplexing (WDM) and Polarization Mode Dispersion (PMD) Performance of a Coherent 40 Gbit/s Dual-Polarization Quadrature Phase Shift Keying (DP-QPSK) Transceiver,” in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2007), paper PDP16.
Future optical transmission technologies involve the utilization of high-order modulation formats to cope with bit-rates per channel of 100 Gb/s and beyond. The most popular solution for 100 Gb/s systems is DP-QPSK (Dual-Polarization Quadrature-Phase-Shift-Keying).
Most of the existing fiber links use 10 Gb/s intensity-modulated (IM) channels. In order to increase the information capacity of an existing fiber link without interrupting existing traffic, new high-speed 100 Gb/s can be added to the link without removing the existing “old” channels. This is a cost efficient solution which permits increasing information capacity without needing to install new fiber cables.
However, 100 G DP-QPSK channels are heavily impaired by the existing co-propagating intensity-modulated channels through fiber nonlinear effects. This strongly limits the performance and transmission reach of this mixed rate links.
Typical solutions to reduce the nonlinear interactions between existing IM channels and new high speed QPSK channels involve adding a frequency guard band between them. Other solution is to reduce the modulation order to (for example) BPSK. This has the disadvantage of reducing spectral efficiency.
Accordingly, there is a need for an alternative modulation format for 100 G channels that present improved performance when such channels co-propagate with existing IM channels.